The present invention relates to an electromagnetic actuator which drives a mechanical element, and more specifically concerns an electromagnetic actuator which drives an intake valve or an exhaust valve of an engine which is used, for example, in an automobile and a boat.
Electromagnetic actuators used to drive the intake and exhaust valves of automobiles, in which an armature (movable iron piece) placed between a pair of opposing springs is driven between one terminal position and the other terminal position by alternatively supplying electric power to a pair of opposing electromagnets, are known from Japanese Patent Applicatikon Kokoku No. Sho 64-9827 and Japanese Patent Application Kokai No. Hei 8-284626, etc.
In common electromagnetic valves, an armature (valve) which is seated as a result of being attracted by one of the electromagnets is released from the seated state by stopping the power supply to the electromagnet and the armature starts to move toward a neutral position at which the opposing force of each of the two opposing springs balances. At a certain timing in synchronization with this movement, electric current is supplied to the other of the electromagnets to attract the armature.
As the armature approaches the other of the electromagnets, the magnetic flux grows abruptly as the work by the attractive force of the other of the electromagnets overcomes the sum of the slight work to draw the armature back by residual magnetic flux of the one of the electromagnets as well as a mechanical loss. Thus, the armature reaches a seated position. As seating takes place, a holding current is supplied at an appropriate timing to maintain the armature in the seated position.
In the valve operating system of an ordinary automobile engine, the amplitude of the displacement of the abovementioned armature between a pair of opposing electromagnets is 6 to 8 mm. The relationship between the attractive force of the electromagnets and the gap between the armature and the yoke is considerably nonlinear, which hinders stable operation.
In an actual valve operation, the mechanical loss varies as the engine load and other factors change so that the magnitude of the mechanical work required for making the armature seat varies (variation in the direction of the spatial axis). Furthermore, as it is not easy to maintain a constant magnetic force for holding the armature in the seated position, there is some variation in the residual magnetic flux when the armature is released. As a result, the dead time (delay: idle time, delay time) from the time when the power supply to the electromagnet is stopped to the time when the armature actually departs the seated position varies (variation in the direction of the time axis).
Conventional electromagnetic actuator driving scheme is essentially unstable with respect to such variations in the direction of the spatial axis and variations in the direction of the time axis.
The driving conditions of the armature in a common conventional electromagnetic actuator will be described with reference to FIG. 4(A). The curve (a) indicates the movement of the armature. The position marked as 0 mm on the left vertical axis indicates the first terminal position. The other or second terminal position is located 7 mm from the first terminal position. When the armature is driven from the first terminal position toward the second terminal position, the armature first begins to move toward the neutral position (where the force of a pair of opposing springs balances) as the current for holding the armature in the first terminal position is cut off. In FIG. 4(A), the armature reaches the neutral position in approximately 3 milli-seconds. When the armature has more or less reached the neutral position, a constant current (b) (2 amperes in the case of the present example) is supplied to the second electromagnet to generate an attractive force (d) that attracts the armature toward the second terminal position. This attractive force (curve d) reaches 600 Newtons at the time of seating, which greatly exceeds the minimum attractive force of 300 Newtons needed for attracting the armature. Curve (f) indicates the level of the minimum attractive force that is required for having the armature seat (this is the same in the following figures).
The voltage applied to the second electromagnet is indicated by curve (c). A rectangular wave voltage with a base frequency of 20 kHz or greater is applied by means of pulse width modulation (PWM) from a 100 V power supply in order to maintain a constant current (b). In the figure, this is indicated as a mean voltage (c) in terms of a moving average. When the armature is seated, the current supplied to the coil is switched to a holding current of approximately 0.5 amperes as shown in the curve (b).
If friction increases for some reason, the attractive force drops. FIG. 4(B) shows the attractive force (d) obtained by supplying a constant current in a case where the friction is 1.5 times the standard friction. In this case, the peak attractive force does not reach the level (f) needed for seating. Thus, the armature cannot reach or seat on the electromagnet. It will oscillate between the two electromagnets by the action of the pair of springs as can be seen from curve (a).
The causes of this problem are thought to be as follows:
1) When the armature is released, the armature is driven toward the opposite electromagnet by the potential energy of the spring. However, as a result of the increase in friction, the proportion of the potential energy of the spring that is converted into kinetic energy of the armature or valve drops. In other words, the distance the armature can travel without power supply decreases.
2) Accordingly, when friction is larger, if current is supplied with the same timing on the time axis, the gap between the armature and the yoke is larger than when there is a standard friction. Since the gap is larger, the rise in the magnetic flux is blunted and the counter electromotive force generated in a driving coil of the electromagnet is also smaller. Consequently, the voltage required to provide the same current flow reduces and the voltage peak lowers. Thus, the flow of electric power (terminal voltagexc3x97current) into the electromagnets from the power supply drops, which further slows down growth of the magnetic flux and the attractive force. This way, when the friction becomes large enough to reach a boundary value, the actuator becomes unable to attract the armature.
In accordance with one aspect of the invention, an electromagnetic actuator comprises two springs which act in opposite directions, an armature which is connected to the springs and is supported in an unactivated state in a neutral position provided by the two springs. The armature is coupled to a mechanical element such as a valve of an engine. The actuator includes a pair of electromagnets that drive the armature between a first terminal position and a second terminal position. It also includes a power supply that controls the voltage supplied to the electromagnet attracting the armature to a constant voltage when the armature is driven from one of the terminal positions to the other of the terminal positions.
As the armature is released from the seated position, it moves toward the electromagnet on the opposite side by the potential energy of the spring. The distance the armature travel reduces with increased friction. Thus, the gap between the armature and the yoke increases causing the counter electromotive force to decrease as described above. In the present invention, the voltage supplied to the electromagnet is maintained at a constant value. Accordingly, if the counter electromotive force decreases, larger current flows in and the power supply (terminal voltagexc3x97current) to the electromagnet increases. As a result, slowdown of growth of the magnetic flux is prevented and a large attractive force grows. Accordingly, increase in the friction is not a problem as in the prior art.
In accordance with another aspect of the invention, the electromagnetic actuator comprises two springs which act in opposite directions, an armature that is connected to the springs and supported in an unactivated state in a neutral position provided by the two springs. The armature is coupled to a mechanical element such as a valve of an engine. The actuator includes a pair of electromagnets that drive the armature between a first terminal position and a second terminal position and a pulse-modulation driver that selectively supplies voltage pulses with a variable duty ratio to the pair of electromagnets.
The actuator further includes a controller that controls the duty ratio such that the electric power needed to generate a sufficient attractive force for attracting the armature is supplied when the armature is driven from one of the terminal positions to the other terminal position. The electric power to be applied can be set beforehand. Accordingly, lowering of the speed of armature movement for soft seating and other controls can be positively performed.